Field of the Invention
The present invention generally relates to a method of etching, and more particularly to an etching method using a silicon nitride stopper for forming a self aligned direct contact to be used in a memory cell such as RAM (Random Access Memory) and SRAM (Static Random Access Memory).
Conventional methods of selectively etching a silicon nitride film include wet etching using hot phosphoric acid, and dry etching using CH.sub.2 F.sub.2 or CH.sub.3 F as disclosed in Japanese Patent Publication No. 6-12765.
However, in a memory cell portion in which a self aligned direct contact is to be formed using a silicon nitride stopper, if the silicon nitride film provided as the stopper layer is removed by wet etching, which will proceed isotropically, a problem will arise that the silicon nitride film cannot be left on the sidewall portion of a pattern as desired.
Further, if a parallel-plate type reactive ion etching system (RIE) is employed with conventionally known reactive gases such as CF.sub.4 and CHF.sub.3, each rate will become higher at the sidewall portion of a pattern than at the bottom of the hole due to sputtering effect, and thus it is difficult to form a sidewall of the silicon nitride film with a necessary height. Moreover, if the above-mentioned dry etching method as disclosed in Japanese Patent Publication No. 6-12765 is employed, as the etch selectivity of a silicon oxide film to a silicon nitride film is 10 or higher, etching will stop at the silicon oxide film located beneath the silicon nitride film, and thus an electrical contact fails.